This invention relates, in general, to an optical transmission plan and optical amplifier circuit therefor, and is particularly, but not exclusively, applicable to a bi-directional wave division multiplexed (WDM) amplifier circuit and a channel assignment plan for optical transmissions therein.
method of increasing the transmission capacity of an optical fibre is to use wavelength division multiplexed (WDM) channels. Indeed, in this respect, optical amplifier designs have been optimised for WDM operation. For example, in uni-directional systems, all the WDM channels will be transmitted through the transmission fibre in the same direction, while an alternative, bidirectional transmission configuration has some channels within the fibre travelling in one direction and other channels travelling in an opposite direction. This latter system configuration offers a number of advantages, especially in relation to the number of redundant transmission terminals required for reliability. As such, optical amplified designs are currently being optimised for bidirectional transmission.
One particular mechanism by which bidirectional transmission are optimised is provided by splitting the available channels into distinct operating bands, one for each direction. These bands are sometimes referred to as the xe2x80x98bluexe2x80x99 and xe2x80x98redxe2x80x99 bands, and can contain varying numbers of channels. For example, the blue band may have channels based within a wavelength range of 1527 to 1540 nanometres (nm), while the red band may have channels based within a wavelength range of 1545nm to 1560nm. Each channel supports information transfer through the modulation of data onto a carrier frequency (generated by a laser). In an optical system, typical modulation techniques include amplitude and phase modulation, and frequency shift keying (FSK).
As will be understood, WDM channel allocation is typically based on the International Telecommunications Union (ITU) standardised wavelength grid, and is therefore subject to specified minimum channel spacing. Specifically, the present ITU standard requires a channel spacing of 100 GigaHertz (GHz), with possible channel combinations using multiples of this 100 GHz channel spacing. In practice (and bearing in mind the constraints imposed by laser light resolution), each band can support sixteen channels having a 100 GHz spacing regime, i.e. thirty-two channels in total.
Unfortunately, as channel spacing decreases, each channel suffers from higher levels of performance degradation due to multi-channel operation. Specifically, the superposition of harmonics results in significant detrimental effects on data integrity within a specific channel. More particularly, in an optical transmission system, densely packed channels exhibit an intermodulation product, known as xe2x80x9cfour-wave mixingxe2x80x9d. Basically, the degradation from the intermodulation product arises from sidebands being superpositioned on or about adjacent optical channels, with each sideband generated as a consequence of the spacing between adjacent channels. Increasing the channel separation therefore improves transmitted signal integrity, but this improved performance only results from sacrificing overall transmission capacity.
One way to maintain the same total number of channels in a bidirectional amplifier, whilst reducing the channel degradation arising from an intermodulation product is to adopt an interleaved channel plan in which adjacent channels propagate in opposite directions. In practice, this means that the channel spacing between adjacent channels in one direction has been doubled, with the effects on adjacent co-propagating channels accordingly reduced. An additional advantage of this scheme arises from the distribution of the xe2x80x9cdead bandxe2x80x9d in which channels cannot be used. Specifically, in relation to a dual band system in which distinct bands are allocated for up-link and down-link transmissions, it is imperative that the bands be separated, (isolated) to prevent corruption of data. Unfortunately, in any attenuated system (as implemented within a filter, for example) isolation is dependent on operating parameters of the actual attenuation devices. In the specific case of filters in an optical system, the filters have a response curve that gradually rises and gradually tails off. Consequently, a minimum extinction zone must exist between the separate channel bands, with the width of the extinction zone necessarily excluding any overlap potentially arising from the lead-in or tail-off profiles of the filter response characteristics. In other words, the rate of increase of attenuation against wavelength (termed xe2x80x9croll-offxe2x80x9d) that is achievable between the channel bands through filtering produces a portion of the bandwidth that cannot be used for communication information; this is the overlapping region of the xe2x80x9cdead bandxe2x80x9d. In an interleaved case, the channel spacing in one direction is doubled and so the filter edges are steep enough not to lose a channel slot due to dead bands. Indeed, in practice, the dead band is distributed across the whole of the operating bandwidth, and so the optical system is able to increase the total number of available channels.
Although it will now be understood that interleaving in an optical transmission system is desirable, the problems associated with the provision of a suitable optical amplifier discourage the implementation of such interleaved schemes. Specifically, the design of a suitable optical amplifier is presently both complex and expensive. Consequently, with the development of a new and improved architecture for an optical amplifier, interleaved optical transmission systems could and would be implemented more frequently, which systems would therefore provide an improvement in the standard, quality and rate of information transfer, as will be appreciated.
EP-A-0 680 168 describes a system and method for optimising spectral efficiency using time-frequency-code slicing. More particularly, through the scheduling of the various speed users within the frequency and time domains, the system and method allocates and makes use of the available spectrum.
EP-A-0 668 675 describes a multi-channel optical fibre communication system in which wavelength division multiplexed channels have simple channel-to-channel spacing which prevent substantial coincidence of 4-wave mixing products with channel carriers.
U.S. Pat. No. 5,390,043 describes an optical heterodyne communication system for transmission of multiple optical channels at different wavelengths, with the optical channels grouped into blocks. Spacing between optical channels with a block is equal to or somewhat greater than the channel bandwidth, while the spacing between blocks is such that when the optical signal is heterodyned with a local oscillator lightwave the interference with a selected optical channel from the adjacent block does not exceed a predetermined level.
According to a first aspect of the present invention there is provided a method of assigning a band of optical channels to an optical fibre arranged to support bidirectional communication in an up-link and a down-link, the band of optical channels having adjacent wavelengths, the method arranged to reduce intermodulation in each of the up-link and down-link by assigning to at least one of the up-link and the down-link at least one pair of channels having adjacent wavelengths in the band of optical channels, wherein assignment of channels to the up-link and down-link are mutually exclusive and each of the up-link and the down-link comprises at least one other optical channel having a wavelength not adjacent to said at least one pair of channels.
In particular embodiments, the optical channels in the up-link and down-link are separated by no more that two contiguous optical channels. Alternatively or additionally, at least one of the up-link and the down-link comprises at least two pairs of adjacent optical channels, the at least two pairs being separated by no more than two adjacent channels.
In a further aspect of the present invention there is provided an optical fibre supporting bidirectional communication on a multiplicity of optical channels having adjacent wavelengths, the multiplicity of optical channels distributed between an up-link and a down-link, the optical fibre supporting at least one pair of channels having adjacent wavelengths in at least one of the up-link and the down-link, wherein channels assigned to the up-link and down-link are mutually exclusive and each of the up-link and the down-link comprises at least one other optical channel having a wavelength not adjacent to said at least one pair of channels, whereby four-wave mixing in each of an up-link and a down-link is reduced.
The optical fibre may, in fact, be one of a plurality of similar optical fibres of an optical communication system.
In still yet another aspect of the present invention there is provided a method of bidirectional transfer of data through an optical fibre, the data conveyed in up-link and down-link directions in the optical fibre in a multiplicity of optical channels having adjacent wavelengths, the multiplicity of optical channels distributed between the up-link and the down-link directions, the method characterised by the steps of: assigning at least one pair of channels having adjacent wavelengths to at least one of the up-link and the down-link, wherein channels assigned to the up-link and down-link directions are mutually exclusive and each of the up-link and the down-link directions comprises at least one other optical channel having a wavelength not adjacent to said at least one pair of channels, whereby four-wave mixing in each of the up-link and down-link directions is reduced; and modulating channels to support simultaneous transfer of the data in the up-link and down-link directions.
In another aspect of the present invention there is provided a circuit responsive to a broadband optical signal supporting a plurality of channels, the circuit comprising: a directional coupler arranged to receive the broadband optical signals and configured to provide an output signal; a filter, responsive to the output signal, having a characteristic arranged to isolate from the output signal at least one desired channel on at least one filtered output; and a reflector coupled to the at least one filtered output and arranged to reflect the at least one filtered output back into the filter such that the at least one filtered output is subjected to a second filtering process that improves isolation of the at least one desired channel.
In yet another aspect of the present invention there is provided a method of isolating at least one desired optical channel from a broadband optical signal applied to a filter, the broadband optical channel supporting a plurality of channels and the filter having a characteristic arranged to isolate the at least one desired channel: filtering the broadband optical signal to produce a filtered output signal containing the at least one desired channel; and re-filtering the filtered output signal in the filter to isolate further the at least one desired channel.